An electrochemical fuel cell is a device that reacts a fuel source with an oxidizing agent to produce an electric current. Commonly, the fuel source is hydrogen gas, and the oxidizing agent is oxygen. An example of a fuel cell utilizing these reactants is a proton exchange membrane fuel cell (PEMFC or PEM fuel cell), in which hydrogen gas is catalytically dissociated in the fuel cell's anode chamber into protons and electrons. The liberated protons are drawn through an electrolytic membrane into the fuel cell's cathode chamber. The electrons cannot pass through the membrane and instead must travel through an external circuit to reach the cathode chamber. In the cathode chamber, the protons and electrons react with oxygen to form water. The net flow of electrons from the anode to the cathode chamber produces an electric current, which can be used to meet the electrical load being applied to the fuel cell by an associated electrical, or energy-consuming, device, such as a vehicle, boat, light, appliance, household, etc.
The fuel cell's ability to transport hydrogen ions across the electrolytic membrane is a function of the hydration of the membrane. In the case of low-temperature fuel cells, such as PEM fuel cells and alkaline fuel cells (AFCs), the ionically-conductive electrolyte is a water-swollen, strongly acidic polymeric membrane (PEMFC) or an aqueous solution of a strong base such as potassium hydroxide (AFC). These ionically-conductive electrolytes are susceptible to drying (losing water) or flooding (absorbing excess water). Either occurrence can lead to poor performance of the fuel cell and premature failure. U.S. Pat. No. 6,059,943, which is incorporated herein by reference in its entirety for all purposes, describes many of the problems related to maintaining a correct water balance in electrochemical devices such as fuel cells.